1. Field of the Invention
The present invention relates to a technique for determining an exposure condition such as focus offset and the like for exposure systems utilized in semiconductor fabrication.
2. Description of the Related Art
Best focus position and effective focus range in exposure are important for exposure systems such as scanners, steppers and the like utilized generally in fabricating semiconductors such as memories, logics, and the like (for example, see Japanese Patent Application Laid-Open No. 2006-41549). The abovementioned effective focus range (to be referred to as the focus margin hereinbelow) is defined to be the range of focus offset such that the line width (the hole diameter in the case of a via hole) of a circuit pattern feature generated as a result of exposure lies within a designed allowable value satisfying the operational specifications as a circuit.
Inspection with a scanning electron microscope (to be referred to as an SEM hereinbelow) is regarded as a typical method for inspecting whether or not the pattern feature is generated within the designed allowable value. However, since it is difficult for an SEM to reach a high throughput in principle, as shown in FIG. 11 for example, the inspection is carried out in practice by sampling five points or so in an exposed shot. On the other hand, scatterometry (Optical CD: to be referred to as OCD hereinbelow) is regarded as a system of measuring wide-area information with a high throughput. However, in order to carry out such measurement, it is necessary to provide a dedicated pattern feature for a certain size of area (approximately 50 μm×50 μm), a library for simulation results, and the like. Therefore, it is difficult in principle to inspect a specific pattern feature within an actual circuit pattern feature.
Further, in LSI fabrication, it is important whether fine and dense pattern features for which that the exposure device may greatly deliver its exposure performance are resolved evenly and just as designed. For example, at the time of exposing vertical line-and-space pattern features, the exposure system per se is optimized with modified or transformed illumination such that those critical pattern features may be exposed with a high degree of accuracy. As a result, the focus margin also becomes comparatively wide with the critical pattern features for which the exposure system is optimized. On the other hand, when focusing attention on pattern features different from the above critical pattern features in the duty ratio of line and space such as so-called isolated pattern features, compared with the focus margin of the critical pattern features for which the exposure system is optimized, the focus margin of the isolated pattern features and the like tends to be narrow. Since these are actual circuit pattern features, the above OCD is not applicable. Further, since a great number of points are to be observed in the pattern feature plane, utilizing an SEM requires an immense amount of time. Further, when the pattern feature shape is not a simple straight line, there are cases that the pattern feature shape cannot be evaluated if a plurality of points are not measured within the visual field, thereby further reducing the throughput of the SEM.
As a means for evaluating best focus, there is a method utilizing an FEM (Focus Exposure Matrix) wafer 100 (see FIG. 10) with shots 101 exposed in sequence on the wafer while changing the focus offset of the exposure device in minute steps. As a control method, the focus margin and the focus offset value for the best focus are calculated by utilizing a measuring device such as the SEM, OCD and the like to monitor the line width (the hole diameter in the case of a via hole) of the pattern feature formed in the photoresist of the FEM wafer 100. However, in the case of the aforementioned pattern features different from the critical pattern features in the duty ratio of line and space, because the control with the OCD is impossible due to the actual circuit pattern features, and utilizing the SEM requires large amounts of time and cost, it is difficult in reality to carry out a routine check. Further, supposing the SEM carried out the control, such method would be considered requiring much time before the measuring result can be fed back.
In contrast, there is a method for monitoring the best focus by detecting a change in polarization due to form birefringence in the pattern feature formed on the wafer surface. According to such method, by finding the peak of brightness (luminance) average for each shot, it is possible to speedily find the best focus. Because it is also possible to measure the best focus in a short time for the critical pattern features in the FEM wafer 100, the measuring result can be rapidly fed back to the process.
However, with respect to isolated pattern features (such as guard pattern features and the like) formed repetitively at intervals of a memory-mat size, because the pitch is greater than that of the critical pattern features, it is difficult in principle to capture the change in polarization. Therefore, diffracted light is utilized because if a pattern feature change occurs such as the pattern feature shown in FIG. 12B where collapse is arising, then a change in diffraction efficiency occurs in the diffracted light from the line-and-space pattern feature or guard pattern feature with respect to the normal pattern feature shown in FIG. 12A. As a result, it is possible to capture the pattern feature change as a brightness change of the diffracted light.